A porous volcanic rock that is light and full of cavities due to expanding gases that were liberated from solution in the lava while it solidified. Pumice is often light enough to float on water. It is usually acid (siliceous) in composition, and is used as an abrasive and for polishing.
Geology and Earth Sciences
Extremely vesicular, frothy, natural glass, having a high (60–75%) silica content and low density. In some cases it will float on water. Usually, but not always, it is of pyroclastic origin. See alsoreticulite; vesicle.
Geography
A very light, fine-grained, and cellular rock produced when the froth on the surface of lava solidifies. See Calcaterra et al. (2007) Geomorph. 87, 3. A pumice raft comprises pumice fragments floating on the sea; one was spotted in 2006 (Geol. News, Nov. 2006).